Fiber lasers are a prominent source of coherent light due to their capability to deliver high power output with good beam quality. Currently, fiber lasers are widely used in industry and scientific research fields. Since their early development, much research and study has concentrated on improving the output power, efficiency, reliability, and beam quality of fiber lasers. To achieve these goals, different wavelengths, materials, cavities, fiber structures, etc., have been implemented.
Total pump absorption plays a role in the overall efficiency of fiber lasers. Accordingly, an accurate measurement of the absorption coefficient is a factor in making fiber lasers more efficient. The absorption coefficient is dependent on the condition of the optical fiber and also its fabrication procedure. Even with the same production process, the absorption coefficient is not constant, hence companies report a wide range of values for the associated absorption coefficients.
FIG. 1 depicts an example of a cutback method according to the conventional art. The conventional cutback method has been widely used to measure the absorption coefficient of doped optical fibers. In this method, doped optical fiber 100 with doped core 102 and cladding 104 is pumped using a laser (not shown), and the pump power transmitted through doped core 102 is measured at points along the length of doped optical fiber 100.
As a first step in the conventional cutback method, doped optical fiber 100 is cut at the 1st cut, as shown in FIG. 1. Then, doped optical fiber 100 is aligned with power meter 106, pumped using the laser, and the pump power transmitted through doped core 102 is measured at the 1st cut using power meter 106.
As a second step in the conventional cutback method, doped optical fiber 100 is cut at the 2nd cut, as shown in FIG. 1. Then, doped optical fiber 100 is realigned with power meter 106, pumped using the laser, and the pump power transmitted through doped core 102 is measured at the 2nd cut using power meter 106.
As a third step in the conventional cutback method, doped optical fiber 100 is cut at the 3rd cut, as shown in FIG. 1. Then, doped optical fiber 100 is realigned with power meter 106, pumped using the laser, and the pump power transmitted through doped core 102 is measured at the 3rd cut using power meter 106.
This process of cutting, aligning/realigning, pumping, and measuring is repeated as required.
As can be observed by a person having ordinary skill in the art (“PHOSITA”), this method amounts to destructive testing of doped optical fiber 100. Thus, once the cutback method is performed on doped optical fiber 100, doped optical fiber 100—as tested—no longer exists to be used.
In addition, the cutting process involves cleaving, stripping, inspecting, polishing, and/or realigning the cut doped optical fiber 100. In particular, for best results, the cut doped optical fiber 100 should be precisely realigned relative to power meter 106 for each measurement, including locating the newly cut end at a same distance ‘d’ from power meter 106. As a result, the repeated realignments introduce error into the conventional cutback method.
Another source of error in the conventional cutback method is the presence of cladding modes. It is virtually impossible to eliminate all of the cladding modes in doped optical fiber 100.
One approach used to reduce the impact of the cladding modes is to cover cladding 104 with index matching gel. However, there are some cladding modes close to doped core 102 that remain unaffected by the index matching gel, and they can propagate through cladding 104, effectively having no interaction with doped core 102.
For passive optical fibers, it can be practical to use the cutback method for optical fibers on the order of a couple of kilometers in length, because the cladding modes can be greatly reduced or eliminated. However, doped optical fiber 100 is intrinsically an absorber, and so a typical length of doped optical fiber 100 used for the cutback method is on the order of one meter, thus the cladding modes cannot be greatly reduced or eliminated. Practically speaking, the presence of cladding modes in doped optical fiber 100 is an unavoidable source of error in measuring the absorption coefficient of doped optical fiber 100.
In the cutback method, all of the pump light should be coupled to doped core 102. Two options for such coupling include splicing with another optical fiber and using a microscope objective.
For the splicing option, it is typically difficult to find a commercially available passive fiber with a core diameter than matches the core diameter of doped core 102 because passive fibers are designed to lase at a lasing wavelength, but most of the time absorption at the pump wavelength is desirable for doped core 102. So a passive fiber should be chosen to carry the pump wavelength.
In one example, when splicing 980-XP (from Nufern, Inc., of East Granby, Conn.) as a passive fiber with SM-YSF-LO-HP (a ytterbium-doped, single-mode, single-clad optical fiber also from Nufern, Inc.) as a doped optical fiber, due to the fact that the mode field diameter at pumping wavelength is not the same in both fibers, the splicing loss is at least about 15%. This means that about 15% or more of the input power is gone, and some of that power couples to the cladding modes. But in an optical fiber length of less than one meter, it is almost impossible to completely remove all of that power from the cladding modes.
For the microscope-objective option, it may not be theoretically possible to couple all of the power into doped core 102. Additionally, it is typically difficult to find a commercially available, highly efficient microscope objective. This means that for the microscope-objective option, there is about 12% coupling loss, and some of that power couples to the cladding modes.
Further problems arise in the cutback method because some doped optical fibers 100, such as ZBLAN (ZrF4—BaF2—LaF3—AlF3—NaF), are mechanically fragile. But the cutback method requires significant handling, movement, and stress to doped optical fiber 100. Thus, handling ZBLAN fibers for the cutback method is extremely difficult.
For measuring the absorption coefficient in a highly doped fiber such as F-DF1100 (from Newport Corporation of Irvine, Calif.), which has a nominal peak absorption at 977 nm of 1,700 decibels per meter (“dB/m”), using the cutback method for the resonant wavelength (peak of absorption) is almost impossible because each piece should be smaller than a millimeter.
For applying the cutback method to certain optical fibers (e.g., ZBLAN fibers, large-mode-area fibers, photonic crystal fibers), because the cutting process involves cleaving, stripping, inspecting, polishing, and/or realigning, different tools may be required. In particular, proper cleaving (e.g., tension, cutting blade angle, straight and flat cleave) is challenging and time-consuming. In addition, ZBLAN fibers, large-mode-area fibers, and photonic crystal fibers are expensive and the cut pieces generally are not useful.
In view of these issues with the cutback method, a non-destructive and accurate method for measuring the absorption coefficient of doped optical fibers would be advantageous.